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Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Derivative [Line Items]
Unrealized gains, before tax	$ (8,081)	$ (8,954)	$ (39,061)	$ 108,508
Unrealized gain, after tax	(6,037)	$ (6,635)	(28,657)	$ 80,404
Income recognized	4,000		9,300
Cash flow hedge gain (loss) to be reclassified			42,000
Cash flow hedge termination gain			7,000
Collateral requirements	72,700		72,700		$ 155,400
Variation margin, amount reducing derivative asset	92,700		92,700		162,700
Foreign Currency Exchange Forwards
Derivative [Line Items]
Notional amount	16,100		16,100		$ 24,100
Cash Flow Hedges
Derivative [Line Items]
Unrealized gains, before tax	488		1,200
Unrealized gain, after tax	$ 364		$ 930

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